1. Field of the Invention
The invention relates to a power assist apparatus and a control method for a power assist apparatus.
2. Description of the Related Art
In manufacturing sites and the like for manufacturing industrial products, a power assist apparatus is used as an apparatus for assisting an operator in the conveyance of heavy loads (workpieces). A workpiece attachment operation includes conveyance and positioning of the workpiece, but when the workpiece is conveyed cooperatively by the operator and the power assist apparatus, the power required to convey the workpiece can be provided by the power assist apparatus. Moreover, by teaching the power assist apparatus, positioning of the workpiece can be performed efficiently. Hence, through use of a power assist apparatus, the workload of the operator can be reduced and an improvement in operability can be achieved. For example, Japanese Patent Application No. 11-245124 (JP-A-11-245124) discloses a power assist apparatus that detects an incline (angle) of a workpiece holding apparatus for holding a workpiece serving as a conveyance subject and drives the workpiece holding apparatus in an up-down direction.
When it is determined in the power assist apparatus described above that the operator has removed his/her hand from the workpiece holding apparatus, respective joint portions of the power assist apparatus are locked to maintain the attitude (incline and conveyance direction) of the workpiece holding apparatus and the workpiece. Further, when it is determined that the operator has gripped the workpiece holding apparatus, the locks on the respective joint portions of the power assist apparatus are released so that the operator can modify the attitude (incline and conveyance direction) of the workpiece holding apparatus and the workpiece freely.
When workpiece positioning is performed using this type of power assist apparatus, the position of the workpiece can be corrected freely through an operation performed by the operator while gripping the workpiece holding apparatus. However, depending on the state of balance (center of gravity position) of the workpiece, the workpiece may tilt unexpectedly about the respective joint portions as soon as the locks on the respective joint portions are released when the operator grips the workpiece holding apparatus. In this case, the operator may be unable to support this displacement, and as a result, the workpiece may contact an attachment subject member, causing either the workpiece or the attachment subject member to break.
Furthermore, allowing the operator to correct the position of the workpiece freely while gripping the workpiece holding apparatus means that the workpiece may wobble during conveyance, depending on the operation of the operator. Therefore, if the operator makes a mistake, the workpiece may contact the attachment subject member or a peripheral portion thereof. Hence, with the power assist apparatus described above, the operator must convey the workpiece while taking great care to adjust the attitude of the workpiece in situations where the workpiece must be conveyed through a gap or the like, for example, and in such cases, a reduction in conveyance efficiency may occur.